Meteorite classification

Meteorite classification
In meteoritics, a meteorite classification system attempts to group similar meteorites and allows scientists to communicate with a standardized terminology when discussing them. Meteorites are classified according to a variety of characteristics, especially mineralogical, petrological, chemical, and isotopic properties.
Terminology
There is no single, standardized terminology used in meteorite classification; however, commonly used terms for categories include types, classes, clans, groups, and subgroups. Some researchers hierarchize these terms, but there is no consensus as to which hierarchy is most appropriate. Meteorites that do not fit any known group (though they may fit somewhere within a higher level of classification) are ungrouped.
Genetic relationships
Meteorite classification may indicate that a "genetic" relationship exists between similar meteorite specimens. Similarly classified meteorites may share a common origin, and therefore may come from the same astronomical object (such as a planet, asteroid, or moon) known as a parent body. However, with current scientific knowledge, these types of relationships between meteorites are difficult to prove.
Traditional classification scheme
Meteorites are often divided into three overall categories based on whether they are dominantly composed of rocky material (stony meteorites), metallic material (iron meteorites), or mixtures (stony–iron meteorites). These categories have been in use since at least the early 19th century but do not have much genetic significance; they are simply a traditional and convenient way of grouping specimens. In fact, the term "stony iron" is a misnomer as currently used. One group of chondrites (CB) has over 50% metal by volume and contains meteorites that were called stony irons until their affinities with chondrites were recognized. Some iron meteorites also contain many silicate inclusions but are rarely described as stony irons.
Nevertheless, these three categories sit at the top of the most widely used meteorite classification system. Stony meteorites are then traditionally divided into two other categories: chondrites (groups of meteorites that have undergone little change since their parent bodies originally formed and are characterized by the presence of chondrules), and achondrites (groups of meteorites that have a complex origin involving asteroidal or planetary differentiation). The iron meteorites were traditionally divided into objects with similar internal structures (octahedrites, hexahedrites, and ataxites), but these terms are now used for purely descriptive purposes and have given way to modern chemical groups. Stony–iron meteorites have always been divided into pallasites (which are now known to comprise several distinct groups) and mesosiderites (a textural term that is also synonymous with the name of a modern group).
Below is a representation of how the meteorite groups fit into the more traditional classification hierarchy:
Chondrites
Carbonaceous chondrite class
CI chondrites (Ivuna-like) group
CM-CO chondrite (mini-chondrule) clan
CM chondrite (Mighei-like) group
CO chondrite (Ornans-like) group
CV-CK chondrite clan
CV chondrite (Vigarano-like) group
CV-oxA chondrite (oxidized, Allende-like) subgroup
CV-oxB chondrite (oxidized, Bali-like) subgroup
CV-red chondrite (reduced) subgroup
CK chondrite (Karoonda-like) group
CR chondrite clan
CR chondrite (Renazzo-like) group
CH chondrite (Allan Hills 85085-like) group
CB chondrite (Bencubbin-like) group
CBa chondrite subgroup
CBb chondrite subgroup
Ordinary chondrite class
H chondrite group
L chondrite group
LL chondrite group
Enstatite chondrite class
EH chondrite group
EL chondrite group
Other chondrite groups, not in one of the major classes
R chondrite (Rumuruti-like) group
K chondrite (Kakangari-like) grouplet (a grouplet is a provisional group with <5 members)
Achondrites
Primitive achondrites
Acapulcoite group
Lodranite group
Winonaite group
Asteroidal achondrites
HED meteorite clan (possibly from asteroid 4 Vesta, also called basaltic achondrites)
Howardite group
Eucrite group
Diogenite group
Angrite group
Aubrite group (enstatite achondrites)
Ureilite group
Brachinite group
Lunar meteorite group
Martian meteorite group (sometimes called "SNC meteorites")
Shergottites
Nakhlites
Chassignites
Other Martian meteorites, e.g., ALH84001
Pallasites
Main group pallasites
Eagle station pallasite grouplet
Pyroxene pallasite grouplet
Mesosiderite group
Magmatic iron meteorite groups
IC iron meteorite group
IIAB iron meteorite group
IIC iron meteorite group
IID iron meteorite group
IIF iron meteorite group
IIG iron meteorite group
IIIAB iron meteorite group
IIIE iron meteorite group
IIIF iron meteorite group
IVA iron meteorite group
IVB iron meteorite group
"Non-magmatic" or primitive iron meteorite groups
IAB iron meteorite "complex" or clan (formerly groups IAB and IIICD)
IAB main group
Udei Station grouplet
Pitts grouplet
sLL (low Au, Low Ni) subgroup
sLM (low Au, Medium Ni) subgroup
sLH (low Au, high Ni) subgroup
sHL (high Au, Low Ni) subgroup
sHH (high Au, high Ni) subgroup
IIE iron meteorite group
Rubin classification
A. E. Rubin (2000) classification scheme:
Alternative schemes
Two alternative general classification schemes were recently published, illustrating the lack of consensus on how to classify meteorites beyond the level of groups. In the Krot et al. scheme (2003) the following hierarchy is used:
Chondrites
Nonchondrites
Primitive
Differentiated
Achondrites
Stony irons
Irons
In the Weisberg et al. (2006) scheme meteorites groups are arranged as follows:
Chondrites
Primitive achondrites
Achondrites
where irons and stony–irons are considered to be achondrites or primitive achondrites, depending on the group.
History
Modern meteorite classification was worked out in the 1860s, based on Gustav Rose's and Nevil Story Maskelyne's classifications. Gustav Rose worked on the meteorite collection of the Museum für Naturkunde, Berlin and Maskelyne on the collection of the British Museum, London. Rose was the first to make different categories for meteorites with chondrules (chondrites) and without (nonchondrites). Story-Maskelyne differentiated between siderites, siderolites and aerolites (now called iron meteorites, stony-iron meteorites and stony meteorite, respectively).
In 1872 Gustav Tschermak published his first meteorite classification based on Gustav Rose's catalog from 1864:
In 1883 Tschermak modified Rose's classification again.
Further modifications were made by Aristides Brezina.
The first chemical classification was published by Oliver C. Farrington, 1907.
George Thurland Prior further improved the classification based on mineralogical and chemical data, introducing the terms mesosiderite, lodranite and enstatite chondrite. In 1923 he published a catalogue of the meteorites in the Natural History Museum (London). He describes his classification as based on Gustav Tschermak and Aristides Brezina with modifications by himself. His main subdivisions were:
Meteoric Irons or Siderites
Meteoric Stony-irons or Siderolites
Meteoric Stones or Aerolites.
He subdivides the "Meteoric Stones" into those that have chondrules (Chondritic Meteoric Stones or Chondrites) and those that don't (Non-chondritic Meteoric Stones or Achondrites). The iron meteorites are subdivided according to their structures as ataxites, hexahedrites and octahedrites. A complete overview of his classification is given in the box below:
Brian Harold Mason published a further revision in the 1960s.
See also
Glossary of meteoritics
Asteroid spectral types
References
External links
Meteorite frequencies by group - National History Museum
Meteorite "family tree" - National History Museum
Meteorite Classification List - Meteorites Australia
it:Meteorite#Classificazione